Recovering aromatic hydrocarbons from mixtures containing the aromatic and non-aromatic hydrocarbons (HCs) can be achieved with liquid-liquid extraction (LLE) or extractive distillation (ED). In ED, a nonvolatile polar solvent is added to an extractive distillation column (EDC) to increase the relative volatility between the more-polar and less-polar components that have close-boiling points. In general, the solvent is added to the upper portion of the EDC and a hydrocarbon (HC) feed is introduced to the middle portion of the EDC. As the nonvolatile solvent descends through the column, it preferentially extracts the more-polar components to form a rich solvent that moves toward the bottom of the EDC while the less-polar component vapor ascends to the top. The overhead vapor is condensed and a portion of the condensate is recycled to the top of the EDC as reflux and the other portion of the condensate is withdrawn as raffinate product. The rich solvent containing the solvent and the more-polar components is fed to a solvent recovery column (SRC) to recover (i) the more-polar components as the overhead product and (ii) the lean solvent (free of the feed components) as the bottom product, which is recycled to the upper portion of the EDC. A portion of the overhead product is recycled to the top of the SRC as the reflux to knock down any entrained solvent in the overhead vapor. The SRC is optionally operated under reduced pressure (vacuum) and/or with a stripping medium to lower the column bottom temperature.
ED processes for recovering aromatic HCs are described in U.S. Pat. No. 7,078,580 to Tian et al., U.S. Pat. No. 4,053,369 to Cines, and F. Lee, et al., “Two Liquid-Phase Extractive Distillation for Aromatics Recovery,” Ind. Eng. Chem. Res. (26) No. 3, 564-573, 1987.
Although the ED process is simpler to implement than the LLE process, ED has a number of crucial operational limitations. For instance, the ED process is more restricted by the boiling range of the feedstock than is the LLE process. In order to achieve acceptable aromatic HCs purity and recovery, the solvent needs to keep essentially all the benzene (which is the heavy key with the lightest aromatic compound boiling at 80.1° C.) at the EDC bottom thereby driving virtually all of the heaviest non-aromatics into the overhead of the EDC. For a narrow boiling-range (C6-C7) aromatic feedstock, the non-aromatic components (the light key) are the C7 naphthenes, such as ethylcyclopentane (boiling point of 103.5° C.). For a full boiling-range (C6-C8) aromatic feedstock, the non-aromatic components (the light key) are the C8 naphthenes, such as ethylcyclohexane (boiling point of 131.8° C.). These compounds become the light key components not only because of their higher boiling points but also because of their stronger tendency to stay with the solvent and aromatic compounds due to their higher polarity as compared to other non-aromatic compounds in the feed. It is much more difficult to recover benzene, toluene and xylenes (BTX) aromatics from the full boiling-range feedstock, such as the full range pyrolysis gasoline, than to recover benzene and toluene from the narrow boiling-range feedstock, such as the C6-C7 reformate. However, even a well defined narrow boiling-range feedstock contains at least two percent of C8 hydrocarbons including C8 aromatics and naphthenes.